1. Technical Field
The present invention relates to a virtual image display apparatus that provides an image formed by an image display element or the like to an observer, and more particularly, to a virtual image display apparatus that is suitable for a head-mounted display mounted to the head of the observer.
2. Related Art
A variety of optical systems have been proposed as an optical system that is assembled in a virtual image display apparatus such as a head-mounted display (hereinafter, referred to as an HMD) mounted to the head of an observer (refer to Japanese Patent Nos. 2746697, 3787399, 4218553 and 4819532).
With respect to the virtual image display apparatus such as an HMD, it is desirable to enlarge a viewing angle of image light and to reduce the weight of the apparatus. In particular, in order to improve the fit, it is important to decrease the thickness in a visual axis direction of an observer and to make the center of gravity close to the observer.
Further, if the field of view of the observer is entirely covered and thus only image light is viewed, an external scene is not viewed by the observer, which may cause uneasiness for the observer. Further, as the external scene and the image to be viewed are overlapped, a variety of new usages of virtual reality is created. Thus, it is desirable to use a display that displays image light in an overlapping manner without obstructing the field of view of the external scene.
Further, in order to improve the fit for the observer and to make the appearance of the form neat, it is generally preferable that the form of the HMD be close to the form of glasses, and it is preferable that the image display device be disposed across the face particularly across the eyes, instead of being disposed above the eyes.
In order to decrease the size of the optical system and to position the image display device according to the position of the eyes of the observer, so as not to obstruct the field of view, it is preferable to first form the display image light in the optical system to form an intermediate image and to use a relay optical system that enlarges the intermediate image to be viewed.
For example, Japanese Patent No. 2746697 discloses a relay optical system that uses a light guiding plate of a parallel plane shape in which a parabolic mirror is provided at an end surface thereof and a projection lens, and forms an intermediate image inside the light guiding plate. However, in the case of the optical system disclosed in Japanese Patent No. 2746697, the projection lens is large in size, which prevents a reduction in size and weight.
Japanese Patent No. 3787399 discloses a relay optical system that uses a light guiding prism having a curved exiting reflective surface and a projection lens. However, the optical system disclosed in Japanese Patent No. 3787399 has no consideration of allowing the observer to view an external scene. In order to view the external scene by applying this optical system, it is necessary to attach a compensation prism onto the reflective surface enlarged over the entire prism and to provide a half mirror on the bonding surface. However, since image light is reflected on the half mirror surface twice, an image becomes very dark.
Japanese Patent No. 4218553 discloses a relay optical system that includes a projection lens, a concave mirror and a light guiding plate. In this optical system, a wavelength plate and a polarization half mirror are combined to enhance the reflection efficiency. However, in order to view an external scene by applying the optical system disclosed in Japanese Patent No. 4218553, it is necessary to attach a compensation lens onto the concave mirror, which generally increases the thickness.
Japanese Patent No. 4819532 discloses a relay optical system that has a short overall length and a compact arrangement due to a bent optical path. However, in the case of the optical system disclosed in Japanese Patent No. 4819532, a convex portion at the center of a prism or a projection lens obstructs the field of view. Further, since image light passing through a half mirror, is returned, and reflected on the half mirror, and is then incident on the eyes, an observed image becomes dark.